In a CNN interview broadcast on August 23, Obama said if the allegations proved true, the situation would "require America's attention."

Syrian opposition sources say between 500 and 1,300 people were killed in the attack.

The government in Damascus denies the allegation. Syrian state television on August 24 accused the rebels of using chemical weapons in the Damascus suburb of Jobar.

In related news, the United Nations disarmament chief arrived in Damascus on August 24 to press Syrian authorities to allow UN inspectors to probe the suspected chemical-weapons attack.

Angela Kane's visit comes after UN chief Ban Ki-moon urged the Syrian government and its opponents to cooperate in the UN efforts to establish an investigation into the alleged attack.

French Foreign Minister Laurent Fabius said the UN experts "must very quickly be allowed to the site to carry out the necessary tests."

"All the information at our disposal converges to indicate that there was a chemical massacre in Syria near Damascus and to indicate that it is Bashar al-Assad's regime that is behind it," Fabius said.

Britain said it believed forces loyal to Syrian President Assad were responsible for the reported attack.